Conventional mobile devices, including mobile phones, smart phones, tablets, and PDAs, typically have a single display screen. Usually this screen is not bi-stable (or non-bistable), meaning the image disappears once the display power is turned off. These non-bistable display screens are often implemented as high-resolution liquid crystal display (LCD) panels. Some eReader devices use bistable display screens. One advantage of the bistable display screen is that the displayed content remains visible after power is removed from the screen.
Given the increasing popularity of such mobile devices, many applications are eliminating the use of paper. For example, smart phones can be used to download and display an airline boarding pass, write a quick note, display an advertisement or coupon, and the like. Mobile devices having a secondary bistable display screen are ideal for such applications. For example, an airline boarding pass can be kept visible in the bistable display screen while the primary display screen is used for other applications, such as making phone calls, web browsing, or the like, or even while the phone is turned off.
FIG. 1 shows a conventional mobile device 100 with a secondary bistable display screen. Referring to FIG. 1, the secondary bistable display screen is implemented as an EPD panel 104. Mobile device 100 also includes an LCD panel 102, a system-on-chip (SOC) 106, a system flash memory 108, a system dynamic random-access memory (DRAM) 110, and other modules 130 such as input/output devices and the like. SOC 106 includes a system processor 140 to control SOC 106 and a memory controller 142 to control memories 108 and 110.
SOC 106 includes an LCD controller 112 to control LCD panel 102. System processor 140 writes display data for LCD panel 102 to an LCD frame buffer 114 in system DRAM 110, and stores control data for LCD panel 102 in an LCD control buffer 116 in system flash memory 108. LCD controller 112 refreshes LCD panel 102 using the display data in LCD frame buffer 114.
EPD panel 104 has been added to mobile device 100 in a conventional manner by adding an external EPD controller 118, which employs a second flash memory 120 and a second DRAM 122. An EPD interface 128, such as an i80 interface, is added to SOC 106 to interface with EPD controller 118. System processor 140 writes display data for EPD panel 104 to system DRAM 110, and then copies that display data, via EPD interface 128, to EPD frame buffer 126 in EPD DRAM 122. EPD controller 118 updates EPD panel 104 using the display data in EPD frame buffer 126. Control data for EPD panel 104 is stored in an EPD control buffer 124 in EPD flash memory 120.
FIG. 2 shows a conventional process 200 for updating EPD panel 104 of mobile device 100 of FIG. 1. Referring to FIG. 2, system processor 140 writes display data to system DRAM 110 at 202, and then copies the display data to EPD DRAM 122 at 204. System processor 140 also writes a display update command to system DRAM 110 at 206, and then copies the display update command to EPD DRAM 122 at 208. EPD controller 118 reads the display data and display update command from EPD DRAM 122 at 210. In response to the command, at 212 EPD controller 118 updates EPD panel 104 using the display data.